Old 1E player now getting back into 5E. What are HIT DICE? I understand hit points and a 3rd level player has rolled 3 times for HP ( I do like the MAX at first level and then roll option) but what are Hit dice and why do they matter?
Hit Points are a pool of damage-soaking ability: damage deducts Hit Points from your total until you are dead. Healing restores Hit Points to the pool.
Hit Dice are how you get Hit Points. Hit Dice are rolled at each level, and your value on the roll is how many Hit Points you gain (plus whatever applicable bonuses). Thus, the total number of Hit Dice you have is equivalent to your Level.
The term “Hit Dice” is used instead of “Level” because monsters often don’t really have levels, because they have no classes. They still have Hit Dice, though, so Hit Dice is a way of referring to level for both players and monsters.
Many things in 5e scale with your Hit Dice – that is, with your level. One of the most important of these is the healing you receive during a short rest: you can “spend” Hit Dice to heal (rolling the Hit Die + your Constitution modifier, and receiving that much healing), which you slowly regain (namely, half your total Hit Dice are recovered with each long rest).